[Risolto] Custom flag issue

Background of the issue:
I am trying to set up a custom flag for my site link nascosto using WPML. I am following the documentation provided at https://wpml.org/documentation/getting-started-guide/language-setup/language-switcher-options/custom-language-flags/.

Symptoms:
The custom flag does not show.

Questions:
Why does the custom flag not show on my site?